ADVANCES IN MANUFACTURING SCIENCE AND TECHNOLOGY
Vol. 37, No. 2, 2013
DOI: 10.2478/amst-2013-0014
RATED STRESS STATE OF FUNCTIONAL SURFACES
BY X-RAY DIFFRACTOMETRIC METHODS
Peter Kurňava, Dana Stančeková, Tomas Nosák, Marian Jurky
Summary
This work deals with investigation of residual stresses during by non-destructive methods turning.
Workpiece material, which was used in experiments was 19 830 steel, which is used for rolling
bearing rings. Residual stress were observed in the application of various cutting parameters by nondestructive methods.
Keywords: x-ray diffractometric method, stress, bearings
Pomiar naprężenia własnego w warstwie wierzchniej metodami dyfraktometrii rentgenowskiej
Streszczenie
W pracy prowadzono badania naprężeń własnych w warstwie wierzchniej w procesie toczenia przy
zastosowaniu metod nieniszczących – metody dyfrakcji rentgenowskiej. W badaniach stosowano stal
19 830 do wytwarzania pierścieni łożysk tocznych. Pomiar wartości naprężenia prowadzono na
powierzchni obrobionej przy stosowaniu różnych wartości parametrów skrawania.
Słowa kluczowe: metoda dyfraktometrii rentgenowskiej, naprężenia, łożyska toczne
1. Introduction
With modern analytical and computational techniques it is usually possible
to assess whether there are tensions in parts. It is not sufficient for a reliable
prognosis of the properties of the component. If during the performance of
functions in the process there was an unexpected failure of components, in many
cases this was due to the presence of residual stresses which can significantly
shorten the life of components. The compressed nature of stress may be
beneficial, for example, in some ways for finishes balling of cyclically loaded
components. But mostly the negative effects of residual stress, especially the
tensile nature, can create cracks, stress corrosion, reduce fatigue resistance.
There are many mechanical, physical, and indirect methods to determine
Address: P. KURŇAVA, Eng., D. STANČEKOVÁ, assoc. prof., PhD. Eng., T. NOSÁK, Eng., M.
JURKY, Eng., Department of Machining and Manufacturing Technology, Faculty of
Mechanical Engineering, University of Žilina, Univerzitná 8215/1, 010 26 Žilina,
Slovakia, peter.kurnava@fstroj.uniza.sk
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P. Kurňava, D. Stančeková, T. Nosák, M. Jurky
residual stresses. But now non-destructive methods become more popular,
because the tested component is fully functional, undamaged and ready for
immediate inclusion in the manufacturing process. The non-destructive
technology is for example X-ray diffractometry. These devices can capture not
only the surface but also inside the studied samples and map the nature of the
tensions that are in it and exactly where they are located. Following to the results
of examinations, the tensions can be removed by heat treatment, annealing or
shot blasting [1÷8].
2. Aim of work
The work process is the issue of residual stresses of the Class 19 tool steel
for cold forming. The measurements were performed using a device to measure
residual stresses iXRD Combo. The work was be divided into parts, which
extend theoretical - practical possibilities, skills and knowledge of the solution
of residual stresses after machining of materials by turning and grinding.
• Monitoring of residual stresses beneath the surface of the machining tools
with defined and undefined geometry of the cutting wedge.
• Monitoring of residual stresses below the surface by grinding sharp and
blunt grinding wheel.
• Design of basic research, application of non-destructive detection
methods.
The performed experiments and evaluated to identify and intensify the
process of turning aside to dry materials characterized by high hardness and
strength will significantly expand the current knowledge of the field.
3. Methods of measuring residual stress
The machined surface is micro inequal. Force effects of the tool and thin
surface layer below the machined surface deforms. As a result of deformation
and heating of the formed surface layer in this layer the stresses and physical mechanical properties were measured. All these factors affect the functional
properties of machined parts, and the quality of machined surface is constantly
monitored and evaluated. Status and quality of the surface layer are very
important aspects, especially for dynamically loaded components, which are
beginning to violate a rule from the surface [7].
Residual stress is one of the manifestations of the machining technology.
The production process remains in parts and structures and operates
continuously without the external load. Its action significantly affects the
functionality of the machined surfaces [8].
Rated stress state...
35
Stress may be:
• tensile – reduces lifetime,
• compressive – increases durability and reduces fatigue.
In clarifying the specific technological causes of residual stresses, the action
of the cutting wedge, and in particular the deformation processes in the cutting
zone, according to Fig. 1. must be taken into account [1].
Fig. 1. Diagram of residual stresses in the surface
layer under the influence of plastic deformation [1].
I – area of compressive stress induced by friction
wedge cutting back on work surface, II – the area
of tensile strain introduced by pressing the plastic
material, III – unaffected area, such as voltage
compensation, I and II
There are many ways for the methods classification.
• depending on the extent to which the integrity of the body under
examination:
− destructive – there is a complete deterioration of the body,
− partly destructive – there is only a partial impairment of the body not
having an impact on its function and reliability, or you can remove the
impairment,
− nondestructive [8].
Between the nondestructive methods for the detection of residual stresses
we include diffraction methods for example, x-ray diffraction.
Diffraction analysis of residual stresses is one of the most promising
methods, although not the easiest in terms of experimental design and
36
P. Kurňava, D. Stančeková, T. Nosák, M. Jurky
interpretation of measurement results. The principle is to determine the lattice
distortion and converting the stress to the linear theory of elasticity [4].
4. Experiments
Cultivated material, used to measure residual stresses was the steel 19 830
BÖHLER S 600. 5 thorns (Fig. 2.) served as rolling bearing rings.
Fig. 2. Thorns on rolling bearing rings
Turning operation was performed on the machine Mazak Nexus 100-II
(Fig. 3). Its main advantage is the extreme versatility and productivity, it
includes 12-digit turret.
Fig. 3. Circular plate SECO RNMN.
060300 S-type CNB300
The individual operations used different cutting plates. In the roughing
circular plate SECO RNMN CNB300 060300 S-type (Fig. 4) was used.
Rated stress state...
37
Cutting conditions for roughing operations:
vc = 100 m · min-1, vc = 150 m · min-1, vc = 200 m · min-1,
f = 0.1 mm, ap = 0.5 mm
Fig. 4. Roughing with a circular plate of KNB
For the finishing operation cylindrical plate SUMITOMO ELECTRIC type
BNC 200 (Fig. 5 and 6) was used.
Fig. 5.SUMITOMO
ELECTRIC type BNC 200
Fig. 6. Finishing with cylindrical plate
Cutting conditions for finishing operations:
vc = 100 m · min-1, vc = 150 m · min-1, vc = 200 m · min-1,
f = 0.05 mm, ap = 0.1 mm
38
P. Kurňava, D. Stančeková, T. Nosák, M. Jurky
Precise machining gradually pushes the technological process of grinding,
because by precise turning we can finish machining with such precision and
quality that corresponds to the grinded surface. For comparison, we applied the
grinded surface quality for those workpieces. The application ran on the sander
TOS-HOSTIVAŘ (Fig. 7). For the experiment we used the blunt and sharp
grinding wheel, with three different speeds of rotation of the workpiece. The
disc rotated in all cases at constant speed 20000 m · s-1, and the workpiece was
rotated at first - 38 min-1, then 95 min-1 and finally 255 min-1. During grinding
and cutting the fluid emulsion H was applied (2%).
Fig. 7. Sander TOS – HOSTIVAŘ
One of the main points of the project is to buy a portable nondestructive
measurement system for measuring residual stresses XRD (Fig. 8) at the
Department of Machining and Manufacturing Technology. Initial experimental
measurement, and validation studies on this machine were made in the city of
Krakow, the Institute of Nonferrous Metals in Gliwice, Division of Light Metals.
iXRD combines versatility, suitability and safety of a fully enclosed housing.
Easily transforms from a laboratory system to a portable system in several
minutes. iXRD is ideal for customers who need a stable laboratory system, but
Rated stress state...
39
Fig. 8. Measurement system for residual
stresses iXRD
also need to be able to work outside, if required. iXRD is available with standard
or wide protective cover for large parts, a flexible goniometer, stand off-road
and laboratory equipment for mapping of residual stresses.
5. The measurement procedure
To measure the residual stresses measurements began by placing the parts
into pad measuring system. A detector arm was focused onto unit area (Fig. 9).
Stresses were measured in the axial and radial directions. After preparing the
parts began to emit X-rays. The device measured the stress to a depth of 12 µm
in the range of angles around 123-171°. Then the computer was depicted with
graphs, which calculated the residual values for shear and residual stresses.
These were processed into tables (see Appendix) and these graphs were created.
Fig. 9. Focusing the machine on component
40
P. Kurňava, D. Stančeková, T. Nosák, M. Jurky
6. Evaluation of stresses in the thorns
Turning Roughing Operation
When using a cutting speed of vc = 100 m · min-1 there were measured
axially compressed nature of the residual stress, whose value has hovered around
-360 MPa, radial residual stress in the application of the same cutting speed
showed -175 MPa (Fig. 10). With increasing cutting speed there was
a reduction of residual stresses in the axial and radial direction. The value of
vc = 150 m · min-1 to reduce tension in the axial direction was compared to
vc = 100 m · min-1 by 40%, in the axial direction decreased by 30%. When using
vc = 200 m · min-1 tension in the axial direction decreased to the values of
vc = 100 m · min-1 by 50%, in the radial direction changed by 250%.
residual stresses - axial
deviation + deviation -
residual stresses - radial
deviation + deviation -
Stresses, MPa
Turning, roughning, residual stresses
Turning surface
Fig. 10. Diagram of residual stresses in rough turning
Rated stress state...
41
Stresses, MPa
Turning, roughning, shear stresses
Turning surface
Fig. 11. Diagram of shear stress in rough turning
When using a cutting speed of vc = 100 m · min-1 there were measured axial
tensile shear stresses of value around 26 MPa, radial shear stresses for the same
cutting speed showed a value -46 MPa. With increasing cutting speed there was
a reduction of shear stress in the axial and radial direction (Fig. 11). The value of
vc = 150 m · min-1, the tensile stress decreased in the axial direction compared to
vc = 100 m · min-1 by 20% in the radial direction, the compressive stress
decreased by 30%. When using vc = 200 m · min-1 the tensile stress in the axial
direction changed by 200% and the change occurred in the radial direction,
where the pressure changed in tensile stress by 300%.
Turning operation finishing
When using a cutting speed of vc = 100 m · min-1 there were measured the
axial tensions, which values correspond to the voltage 850 MPa. Radial residual
stress for the same cutting speed showed a value 580 MPa. The value vc = 150
m · min-1 the tension in the axial direction increased compared to vc = 100
m · min-1 by 2%, in the radial direction increased by 20%. When using vc = 200
m · min-1 tension increased in the axial direction to the values of vc = 100
m · min-1 by 250%, in the radial direction of the residual stress increased by 25%
(Fig. 12).
42
P. Kurňava, D. Stančeková, T. Nosák, M. Jurky
residual stresses - axial
Turning, finishing, residual stresses
residual stresses - radial
deviation -
deviation +
deviation -
Stresses, MPa
deviation +
Turning surface
Fig. 12. Diagram of residual stresses in turning finishing
shear stresses - axial
shear stresses - radial
deviation +
deviation+
deviation -
deviation -
Stresses, MPa
Turning, finishing, shear stresses
Turning surface
Fig. 13. Diagram of shear stresses in turning finishing
When using a cutting speed of vc = 100 m · min-1 there were measured axial
shear stresses which value was 58 MPa, radial shear stresses for the same cutting
speed showed a value of 60 MPa. The value vc = 150 m · min-1 decrease in the
axial direction compared to vc = 100 m · min-1 by 5%, in the radial direction
of tensile stress increased by 30%. The application vc = 200 m · min-1 decreased
the value of shear stress in the axial direction compared to the values of the
vc = 100 m · min-1 by 10%, in the radial direction, the tensile shear stress
increased by 55% (Fig. 13).
Rated stress state...
43
7. Roughing grinding operation with a sharp wheel
When applying a rotational speed of parts vc = 38 min-1 there were
measured axial residual stresses 840 MPa and radially residual stresses
620 MPa. With increasing rotational speed, there was a reduction of residual
stresses in the axial and in the radial direction. The value of = 95 min-1 reduces
tension in the axial direction to 38 min-1 by 30%, in the radial direction
decreases by 35%. When a = 255 min-1 reduces tension in the axial direction to
= 38min-1 by 40% and in the radial direction by 80% (Fig. 14).
The shear stress were measured by tensile stresses. When applying
a rotational speed of parts = 38 min-1 there were measured axial shear stresses,
the value was around 17 MPa, radial shear stresses applied at the same rotational
speed showed a value of 10 MPa. With increasing rotational speed increased
shear stress in the axial and radial direction. The value vc = 95 min-1 is increased
tension in the axial direction compared to 38 min-1, by 25%, in the radial
direction to increase the voltage by 100%. When there was a = 255 min-1 tension
in the axial direction increased by 52% and in the radial direction by 200%
(Fig. 15).
residual stresses - axial
deviation +
deviation -
residual stresses - radial
deviation deviation +
Stresses, MPa
Turning, residual stresses
Turning surface
Fig. 14. Diagram of residual stresses in grinding roughing with a sharp wheel
44
P. Kurňava, D. Stančeková, T. Nosák, M. Jurky
shear stresses
shear stresses - axial
shear stresses - radial
deviation +
deviation
+
deviation -
deviation -
Stresses, MPa
Turning,
Turning surface
Fig. 15. Diagram of shear stresses in roughing grinding with a sharp wheel
Rough grinding operation with a blunt wheel
When applying a rotational speed 38 min-1 there were measured axial
residual stress 190 MPa tensile, stress of 120 MPa. With increasing rotational
speed, there was an increase of residual stresses in the axial and radial direction,
there is a change in supply pressure. The value of 95 min-1 increased the stress
in the axial direction compared to the 38 min-1 by 68%, in the radial direction
decreased by 216% (Fig. 16).
Stresses, MPa
Grinding, roughing with sharp Wheel, residua stresses
Grinding surface
Fig. 16. Diagram of residual stresses in grinding roughing with a blunt wheel
Rated stress state...
45
When using a = 255 min-1 tension in the axial direction decreased to
a = 38 min-1 by 78% in the radial direction compared to the first rate decreased
by 240% (Fig. 17).
The measured shear stresses were measured by tensile stresses. For a rotational speed of parts 38 min-1 there were measured axial shear stresses, the
value was around 13 MPa, radial shear stresses for the same rotational speed
showed the value of 9 MPa. With increasing rotational speed the shear stress in
the axial and radial direction increased.
shear stresses - axial
shear stresses - radial
deviation +
deviation +
deviation -
deviation -
Stresses, MPa
Grinding, roughing with sharp wheel, shear stresses
Grinding surface
Fig. 17. Diagram of shear stresses in roughing grinding with a bunt wheel
The value vc = 95 min-1 is increased in the axial direction compared to the
38 min-1 by 70%. In the radial direction to increase the voltage by 11%. For
a = 255 min-1 there is an increased tension in the axial direction by 138% and in
a radial direction by 13%.
Rough grinding operation with a very blunt wheel
When applying a rotational speed of parts 38 min-1 there were measured
axial residual stress values around 680 MPa and radial residual stress showed
a value of about 550 MPa. With increasing rotational speed there was
a reduction of residual stress in the axial and radial direction. The value of
vc = 95 min-1, reduced tensions in the axial direction compared to 38 min-1 by
15%, in the radial direction decreased by 4%. When using a = 255 min-1 tension
decreased in the axial direction to the values for the application vc = 38 min-1 in
the axial direction by 30% in the radial direction by 4% (Fig. 18).
46
P. Kurňava, D. Stančeková, T. Nosák, M. Jurky
Stresses, MPa
Grinding, roughing with sharp wheel, residual stresses
Grinding surface
Fig. 18. Graph of residual stresses in grinding of roughing very dull blade
When applying a rotational speed of parts = 38 min-1 there were measured
axial shear stresses, the value was around 20 MPa, radial shear stresses applied
at the same rotational speed of components showed a value of 10 MPa. With
increasing cutting speed increased shear stress in the axial and radial direction.
The value of vc = 95 min-1 increased tension in the axial direction compared to
38 min-1 by 10%, in the radial direction voltage is increased by 280%. When
using a = 255 min-1 tension increased in the axial direction by 50% and the
change occurred in the radial direction, where the compressive stress in the
radial direction changed by 200% (Fig. 19).
shear stresses - axial
shear stresses - radial
deviation + deviation -
deviation +
deviation -
Stresses, MPa
Grinding, roughing with sharp wheel, shear stresses
Grinding surface
Fig. 19. Diagram of shear stresses in roughing grinding with a very blunt wheel
Rated stress state...
47
Grinding operation finishing
For a rotational speed of 38 min-1 there were measured axial residual stress
of about 890 MPa and radial residual stress of approximately -580 MPa. With
increasing rotational speed, in the axial residual stress in the radial direction
increased. For vc = 95 min-1 tension in the axial direction compared to a = 38
min-1 in the radial direction increased by 10%. When using a = 255 min-1 tension
increased in the axial direction to the value of vc = 38 min-1 by 20% in the radial
direction and compared to the first speed increased by 15% (Fig. 20).
Stresses, MPa
Grinding, roughing with sharp Wheel, shear stresses
Grinding surface
Fig. 20. Diagram of residual stresses in grinding finishing with a sharp wheel
Stresses, MPa
Grinding, roughing with sharp Wheel, shear stresses
Grinding surface
Fig. 21. Diagram of shear stresses in grinding finishing with a sharp wheel
48
P. Kurňava, D. Stančeková, T. Nosák, M. Jurky
When measuring the shear stress only tensile stresses were measured for all
the rotational speed of the workpiece. When applying a rotational speed of
38 min-1 there were measured axial shear stress, which value fluctuated around
the value of 54 MPa, radial shear stresses for the same rotational speed of
components showed value 30 MPa. With increasing rotational speed there was
a reduction of shear stress in the axial and radial directions. The value of vc =
= 95 min-1 reduced tension in the axial direction compared to 38 min-1 by 10%,
in the radial direction increased the voltage by 25%. The application of a =
255 min-1 reduced tension in the axial compared a = 38 min-1 direction by 15%
in the radial direction, the voltage was reduced by 70% (Fig. 21).
8. Conclusion
After the machining process, the inner stresses appear. These stresses can
be tensile or compressive. The preferred option, in all circumstances, is the
compressive one. That component is used as a rolling tools which acts on the
surface of a material as a ductile strength. The residual tensile strresses have
a negative effect on the functional properties of a material. They have a great
impact on the spread of cracks in a material. A compressive stress is appropriate
because the distance between atoms are very small, they tend to associate and
act against cracks in the workpiece. The value of compressive stress should not
reach high values, for example 2 GPa. The optimal value of the residual stress
varies is in the range of 500-700 MPa. The residual stress in a given case should
not exceed 1000 MPa, the most extreme value for the steel is 2550 MPa. In this
experiment, the values of residual stresses in some places were 2 GPa. Such
a large residual stress was caused by previous use of the rolling mandrel. For this
thorn in the process worked great forces that have a large impact on the results
of measurements of residual stresses in the experiment. The grinding process
was detected by measuring the residual stress that occurred in the nature of the
residual tensile stress as compared with turning, where the dominant character of
residual stress is compressive. For this reason, the grinding process gradually
pushed out of the technological process to complete and accurate turning is
replaced to prevent tensile residual stresses, reduce the economic cost of
acquisition of grinding wheels and sanding their profiles, as well as to reduce the
time needed for working components in the required quality. Tools with a welldefined geometry are economically more acceptable for use in the machining of
complex shapes and complex components such as the use of grinding
technology.
In production of the components, values of cutting tool wear, which
increase the wear and tear, negatively affect the quality of the machined surface,
i. e. high roughness (Fig. 22). Such negative effect is seen in places where high
Rated stress state...
49
temperature is induced by machining and consequently a high voltage is
generated. It is inappropriate to apply too high or too low cutting speeds when
machining. The results suggest medium cutting speed to achieve optimal results
in the accuracy and surface quality. The processed results suggest that turning
is more favorable, because the roughness is comparable, or in some cases lower
than in grinding. Turning achieved residual stress nature of the pressure to
a greater extent than in grinding, where the tensile residual stress occurred more
frequently.
Fig. 22. Poor surface quality due to high wear of cutting plates
This work is related to the project with the University of Zilina, 2009/2.2/04-SORO
OPVaV number (26220220101). His name is' Intelligent system for nondestructive
evaluation technologies for functional properties of components of X-ray difractometry.
Our aim is to transform the new non-destructive technologies for knowledge transfer to
industry practice in the evaluation of functional properties of the surface and subsurface
layers of nondestructive techniques.
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Received in November 2012